Digital information recording apparatuses such as digital VTRs and MD (Minidisc) recording and reproducing apparatuses have become widespread, and further DVD (Digital Video Disc or Digital Versatile Disc) apparatuses each equipped with a recording function have also come on the market. In these digital information recording apparatuses, various additional information signals are allowed to be record ed in the wake of digital video signals, digital audio signals and further data for computers and the like, which are main information signals.
In this case, this additional information signal is a digital signal, and is added to a digital information signal as a signal to be recorded in such an area as is regionally distinguished from the digital information signal, such as a header portion to be added to data of, for example, a digital information signal in units of blocks, and an area for other TOC (Table of Contents).
As described above, in the case of a system of transmitting by superimposing additional information on a conventional main information signal for recording among others, the additional information sign al is not directly superimposed on a digital information signal such as a header portion, but is arranged to be added to an indirect portion. For this reason, the additional information signal is comparatively easily removed by filtering or tampering, thus possibly making it impossible to detect a necessary additional information signal by a recording apparatus or a reproducing apparatus. Particularly, if control information to prevent unauthorized duplication, copyright information or the like is added as an additional information signal, the removal of such an additional information signal will bring about such a state that the initial object cannot be achieved.
Also, in the case of addition of an additional information signal to such an indirect portion as described above, the additional information signal will be lost when the digital information signal is converted into an analog signal because only the main information signal can be obtained. This means that even if such a measure as to prevent unauthorized duplication of a digital information signal is taken by superimposing such a duplication-preventive control signal as described above as an additional information signal, such a situation that the duplication-preventive measure counts for nothing any longer will develop when converted into an analog signal.
As a superimposition system for an additional information signal capable of solving the problem on the lost additional information signal and the problem when the additional information signal is converted into an analog signal as described above, the present applicant previously proposed a system for spectrum spreading an additional information signal such as the duplication-preventive control signal, and superimposing this additional information signal thus spectrum spread on a video signal to digital-record or analog-record the video signal (U.S. patent application Ser. No. 08/755,101, U.S. Pat. No. 5,982,977).
In this system, a code (hereinafter, referred to as PN code) of, for example, PN (Pseudorandom Noise) series, which is used as a spread code, is caused to be generated at a sufficiently early period, and this code is spectrum spread by applying it to the additional information signal, whereby an additional information signal such as a duplication-preventive control signal of a narrow band on a high level is caused to be converted into a signal of a wide band on an infinitesimal level which does not affect the video signal. Thus, this additional information signal is spectrum spread, that is, the spread spectrum signal is superimposed on the analog video signal to record on a recording medium. In this case, a video signal to be recorded on the recording medium may be either analog or digital.
In this system, the additional information signal such as the duplication-preventive control signal is superimposed at the same time and within the same frequency as the video signal, and therefore, it is difficult, for, for example, a person who intends to duplicate illegally to remove the duplication-preventive control signal superimposed from the video signal. On the other hand, it is possible to utilize by detecting an additional information signal such as the duplication-preventive control signal superimposed by inverse spread spectrum.
Since the additional information superimposed on the video signal is not removed from the video signal when the video signal is reproduced, it is necessary as described above to superimpose the additional information on the video signal on an infinitesimal level which does not affect the reproduced image of the video signal. However, the infinitesimal level must be a level on which the additional information can be detected.
As described above, it is possible to reliably supply the duplication-preventive control signal together with the video signal to a recording apparatus side, and on the side of this recording apparatus, it is also possible to detect this duplication-preventive control signal for reliably performing duplication control corresponding to the duplication-preventive control signal detected.
In the case of recording the video signal particularly as a digital signal, the video signal is converted into one or more blocks in units of plural pixels to be subjected to a compression process in the block unit. As this compression technique, the MPEG2 system or the like is generally used. In this MPEG2, the video signal is converted from a time-base area signal to a DCT coefficient in a frequency area and is compressed by means of the DCT process in block unit. On the reproduction side, the inverse DCT process is performed so that the data of the DCT coefficient is returned to the digital video signal in the time-base area.
It is, of course, possible to superimpose an additional information spectrum spread as described above also on the video signal transmitted accompanied by such data compression. As regards the additional information superimposed, it is generally considered to detect after returned from the DCT coefficient to the time-base area signal.
However, in a case where the additional information cannot be detected unless it is returned to the time-base area signal in advance as described above, there arises inconvenience that it becomes possible to duplicate digital information at a stage of DCT coefficient in a compressed bit stream state even if the additional information in question is, for example, information for duplication-preventive control and the content of the information is prohibited duplication.
In the light of the above-described points, it is an object of the present invention to provide a method and an apparatus capable of reliably detecting additional information superimposed even if an information signal compressed using DCT is not returned to the time-base area signal.